1. Field of the Invention
The present invention relates to a frame buffer for use in color imaging and, more particularly, to a frame buffer having a plurality of levels of resolution.
2. Description of the Related Art
In generating color pictorial images, a large number of colors and moderate spatial resolution are generally required to achieve a high-quality image. Because the eye can detect approximately 100 intensity levels, i.e., for three color separations, seven bits per color separation per pixel, imaging systems should support at least this number of intensity levels. Generally, however, imaging systems support 256 different intensity levels. The 256 intensity levels supported by an imaging system performing three color separations for a full-color image correspond to eight bits per color separation, i.e., twenty-four bits per pixel. Accordingly, for high-quality renditions of real-life scenes, an imaging system supporting at least 100 intensity levels detectable by the eye requires less than 200 pixels per inch to achieve an image having a sufficient level of spatial resolution.
When material such as textual material and synthetic graphic material is being imaged, the accuracy of color is not nearly so important to achieve a high-quality image, particularly since the color used is generally a constant black. High spatial resolution is, however, needed to provide images having crisp, clear edges.
A desirable imaging system would support high-quality color pictorial images, synthetic graphic material and textual material. Heretofore, such an imaging system would necessarily have both a large color space, i.e., many bits per pixel, and a high-resolution level, i.e., many pixels, thus resulting in requirements for extensive memory capability and high bandwidth.
One known system separates pictorial material from textual and graphic material. The system processes the pictorial material in a different manner from the textual and graphic material and combines the results in a final imaging stage. Images commonly, however, incorporate combinations of types of material. For example, an image can comprise a picture of text. Accordingly, the separation and combination performed by the system can be extremely complex, particularly when there is an overlap in the types of material being imaged.
One known technique used for providing high-resolution for edge detail while providing lower resolution for object interiors is the method of quad-trees. This technique represents the image as a tree structure where each level of the tree expands to twice the resolution of the parent level. However, quad-trees require tree traversal to access pixels rather than the simple indexing of a frame buffer.
U.S. Pat. No. 4,782,399 to Sato, discloses an image processing apparatus having image input systems for input of image data of high and low-resolution. A processor discriminates an edge block in the image data, and a filter performs edge detection of an output from a low-resolution image input system. A signal selection circuit selects a signal from high-resolution and low-resolution image input systems and produces the selected signal as an output signal so as to reproduce optimum quality images for all types of original images including character and half tone images. The Sato apparatus thus processes the high resolution and low resolution image data differently. The Sato apparatus, accordingly, is complex in operation.
U.S. Pat. No. 4,703,363 to Kitamura discloses an apparatus for smoothing jagged border lines of an image by providing weight coefficients to a center pixel and surrounding pixels. Values are then obtained for designating middle level densities to be used for the smoothing in accordance with the sum of the coefficients. The apparatus does not provide an imaging system which supports pictorial material, synthetic graphic material and textual material without requiring extensive memory capability and high bandwidth.
U.S. Pat. No. 4,618,990 to Sieb, Jr., et al discloses a method of edge enhancement of digitized fluorographic images by defining frequency components to be enhanced to sharpen images. The frequency components correspond to the frequency response of the edge enhancement filter. An edge map results which corresponds to frequency components at edges which are added to corresponding pixels in the original image, resulting in sharpened edges. The method disclosed by the reference thus requires independent processing at edges and subsequent addition of a resultant edge map in the original image.
U.S. Pat. No. 4,682,869 to Itoh et al discloses an image processing system allowing communication with input and output devices having varying resolutions by converting input images into images having any desired level of resolution up to that of the input. The system thus requires a plurality of devices having varying resolutions to achieve a desired level of resolution in a resultant image.
An imaging system is desired which has the capability to support pictorial material, textual material and synthetic graphic material without requiring both a large color space and a high-resolution. Such an imaging system should generate high-quality images without significantly increasing the complexity of the system.